uapi headers: simplify the <linux/compiler.h> stuff.

We don't really need <linux/compiler.h> and <linux/compiler_types.h>. We
already have a mechanism to remove unused macros, so let's do that. We
don't currently have a way to remove unused #includes, so we still need
<linux/compiler.h> and <linux/compiler_types.h> files (but I've clarified
the comments in them).

I've kept the empty definitions of `__user` and `__force` for source
compatibility. (We had one security test at least that was assuming
a kernel struct definition will "just work".)

Bug: http://b/262917450
Test: treehugger
Change-Id: Iacbbbc1aeef9a4fac52dabd7811ab875cc267d4f
diff --git a/libc/kernel/uapi/linux/nfs4_mount.h b/libc/kernel/uapi/linux/nfs4_mount.h
index 0f794dd..9b9c9f8 100644
--- a/libc/kernel/uapi/linux/nfs4_mount.h
+++ b/libc/kernel/uapi/linux/nfs4_mount.h
@@ -21,7 +21,7 @@
 #define NFS4_MOUNT_VERSION 1
 struct nfs_string {
   unsigned int len;
-  const char __user * data;
+  const char  * data;
 };
 struct nfs4_mount_data {
   int version;
@@ -38,10 +38,10 @@
   struct nfs_string mnt_path;
   struct nfs_string hostname;
   unsigned int host_addrlen;
-  struct sockaddr __user * host_addr;
+  struct sockaddr  * host_addr;
   int proto;
   int auth_flavourlen;
-  int __user * auth_flavours;
+  int  * auth_flavours;
 };
 #define NFS4_MOUNT_SOFT 0x0001
 #define NFS4_MOUNT_INTR 0x0002